Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual die from the finished wafer and packaging the die to provide structural support and environmental isolation.
MOSFETs are commonly used in electronic circuits, such as communication systems and power supplies. Power MOSFETs are particularly useful when used as electric switches to enable and disable the conduction of relatively large currents. The on/off state of the power MOSFET is controlled by applying (or removing) a triggering signal at the gate electrode. When turned on, the electric current in the MOSFET flows between the drain and source.
In the design of power MOSFETS, there is a trade-off between maximizing the breakdown voltage and minimizing the on-resistance. A high drain-to-source breakdown voltage (BVDSS) is desired because it indicates the maximum reverse voltage that can be applied across the device operating as a gated switch, i.e., without causing an exponential increase in current. At the same time, static drain-source on-resistance (RDSON) should be minimized to reduce power loss and heat dissipation when the switch is turned on. The BVDSS can be increased by providing a thicker or lightly doped drift region, however, these features lead to higher RDSON.
To further illustrate the BVDSS and RDSON tradeoff, a conventional high voltage n-channel laterally diffused metal oxide semiconductor (LDMOS) is shown in FIG. 1. Semiconductor device 10 is built on a p-type substrate 11, and includes a highly doped n+ region at drain 13, laterally diffused drift region LDD2 adjacent the n+ drain region, and a lightly doped laterally diffused drift region LDD1 extending from the LDD2 region to gate 15. A highly doped n+ source region is formed at source 17. P+ plug is formed below the n+ source. A p-channel beneath gate 15 separates the source and drain. One difficulty with the design is that when the gate is energized, a high electric field occurs at the gate edge of LDD1, resulting in a lower BVDSS. One way to reduce the electric field uses a lower doping concentration in LDD1, but that approach also increases RDSON.
In addition to optimizing BVDSS and RDSON, other challenges in MOSFET design (as well as the design of other types of semiconductors) include miniaturizing the devices, improving switching frequencies, reducing switching losses, and reducing undesirable phenomena such as floating body effects, punch-through, and parasitic capacitance.
One way of reducing parasitic capacitance involves silicon-on-insulator (SOI) technology. SOI refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics. SOI-based devices differ from conventional silicon devices in that the silicon junction lies above a buried oxide (BOX) layer, typically silicon dioxide. FIG. 2 shows a typical SOI substrate 20. The base silicon or handling layer 21 serves primarily to provide structural support. BOX layer 23 is an insulating layer, and silicon layer 25 is the active layer of the device which may be customized using known fabrication technologies to create semiconductor dies.